"I've never told anybody this, but I found the whole letting go of the characters for X-Men 3 so traumatising that a friend, who shall remain nameless, had to take me to a secret location and show me the reels so far," Singer told Metro. "Just so I could make my peace with it."

Singer is now returning to the series for the first time since X2, directing the upcoming X-Men: Days of Future Past.

The film, based on the 1981 comic book storyline by Chris Claremont and John Byrne, will bring together original X-Men cast members (Ian McKellen, Patrick Stewart and Anna Paquin among others) and X-Men: First Class's Michael Fassbender, James McAvoy and Jennifer Lawrence.

Claremont has said that he expects the screenplay, which is written by Simon Kinberg and Matthew Vaughn, to include significant changes from his original storyline.

Watch Bryan Singer and James McAvoy discussing X-Men: Days of Future Past with Digital Spy below: